1. Field of the Invention
The present invention relates to a calibration technology in a parallel kinematic mechanism which requires no measurements after posturing.
2. Description of the Related Art
A parallel kinematic mechanism which is provided with a base and an end effecter connected in parallel by a plurality of driver shafts has higher rigidity and accuracy in posturing than a mechanism which is provided with a cantilever. The Stewart platform is a typical example of the parallel kinematic mechanisms. The Stewart platform has straight driver shafts or struts which are expanded or contracted to control the posture (including a position and an orientation) of the end effecter. For the highly accurate posturing, however, it is necessary to calculate accurate kinematic parameters such as the length of a strut, coordinates of a joint connecting a strut and a base, and a joint connecting a strut and the end effecter. This operation is known as parallel kinematic mechanism calibration which is studied by various research institutes of the industry, the government, and the academy.
Generally, such calibration requires solution of multiple simultaneous equations whose number is the same as those of the parameters. To this end, it is necessary to place the end effecter in a determined position and a determined orientation, and determine position information (X, Y, Z) and orientation information (A, B, C) in the determined state.
Japanese Unexamined Patent Publication No. 2002-91522 discloses a technology whereby the end effecter is moved in a circle in a given posture, and a radial error in the locus of the circular movement is measured by a double ball bar (DBB) type of range meter, and kinematic parameters are then calculated based on thus-obtained measurement values. Also, Japanese Unexamined Patent Publication No. 2003-200367 discloses a technology that multiple simultaneous equations are separated into eleven or more equations showing relationships between the end effecter position and the kinematic parameters, and one equation showing a relationship between the end effecter orientation and the kinematic parameters, and these equations are solved to calculate kinematic parameters to execute calibration.
However, it can be seen that the former prior art documents refer to the fact that at least one kinematic parameter cannot be acquired. Also, the technology of the latter prior art document requires measurement after posturing, and in particular requires at least one measurement in the posture that involves difficult measurement.